Where high bit rate data signals are transmitted through band-limited channels, it is common practice to employ an adaptive transversal equalizer to combat cross-talk introduced by the band limiting process. A conventional technique of the controlling operation of an equalizer wherein a reference signal is used involves the training of weights by which received signals are modified. Many communication systems employing adaptive equalizers require a period of time devoted to this training of the equalizer weights, during which period no data is transmitted, but rather a perfect replica of a prescribed transmitted signal is regenerated at the receiver as a reference for the equalizer.
It is often desirable to continue to adjust or update the weight values of the equalizer after the training mode has been completed and data is being transmitted and received. A common approach to handling this problem is to use the decided data values as reference signals for the equalizer. A schematic diagram of an equalizer arrangement wherein this technique is employed is shown in FIG. 1.
In accordance with this approach, multiple phase input signals, for example QAM signals, are supplied to an adaptive equalizer 10, from which phase quadrature X and Y components of a received signal vector are derived. These X and Y components are then analyzed in appropriate decision logic circuit 11, from which there are obtained estimates R.sub.EST and P.sub.EST of the magnitude and phase of the received signal vector, respectively. From these estimates, a pair of error signals .DELTA.X and .DELTA.Y, determined by the degree of amplitude and phase error, relative to decided transmitted signal node values, are generated and fed back to equalizer 10 to update the weight values of the equalizer.
If the error rate is low, this approach provides a nearly perfect reference for the equalizer. However, a severe problem arises in multiple phase signal systems wherein phase node slips may occur. While such slips can be taken into account in data derivation by differential encoding of the data, they insert inadmissable errors in the reference signals for the adaptive equalizer.